AFAIK, AotC and RotS, though shot in 1920 by 817, would’ve been finished at cinema 2k res for scope (2048 by 858) and had their vfx shots rendered out at that res as well. For the entirely cgi shots, this would’ve been relatively straightforward. Shots involving live action footage were presumably upscaled very slightly, but I could be wrong about that. It’s possible that ILM worked on all the live-action shots at their native res and they weren’t upscaled to “true 2k” until the final DI pass.
The other thing to keep in mind is that the blu-rays are not the best these movies can look. Lucasfilm would presumably have the 2k, 10/12-bit, 4:4:4, DCI-P3 digital cinema source master files on a hard drive somewhere in the vault whereas the blu-rays are 1920, 8-bit, 4:2:0, rec 709 hd video. These movies can look a lot better than their current home video iterations. Even if we take the limitations of 1080p bd into account, I’d still really love to know how AotC turned out looking so tinted and blurry.
There’s actually already been at least one movie shot in 1920:1080 released on UHD, Resident Evil: Afterlife. The pixel count of the cameras used on AoTC/RotS isn’t the problem, it’s more the other specs like color sub-sampling and whatnot. But like I said, these movies were still finished at 2k.
TPM, though shot on film, had vfx added to almost every last shot. This would’ve been done at 2k. So, the “original negative” for all intents and purposes are the finished rendered frames that were filmed out to actual 35mm negative 20 years (and at least a couple weeks) ago today. The digital filmout tapes were still readable in 2011 and were used to rebuild the movie.
It’s worth noting that 2k vfx were the norm up until only very recently. Even a movie like After Earth, which was shot and finished in 4k, only had its vfx rendered at 2k. The effects shots for Blade Runner 2049 (which Harmy worked on) were finished at 3.4k for the live action shots (to match the native res of the cameras) and 4k for the fully cg shots. TLJ had its vfx rendered out at 2048 by 1718 so that when it was “unsqueezed” by 2x (just like the cinemascope camera footage) it would be full 4096.